Magnetic heads adapted for use with tape cartridges are well known. Such heads are composed of metals where minimum practicable gap size is limited to forty microinches. Consequently, metallic head structures are excluded from use with cartridges with tapes having more than 12,000 flux reversals per inch. The prime reason for this is that the metallic heads become rounded with use causing degradation in performance. Such degradation is not avoided by use of conventional harder materials such as mumetal or metglass which are relatively inexpensive. Ferrite composite materials which are sufficiently hard to avoid such wear are too costly and difficult to work with during manufacture.
The problem of wear resistance is particularly acute with video cassette recorders (VCR's) which employ chromium dioxide tapes which are highly abrasive. VCR's use ferrite heads which are elevated from the tape surface (flying) during operation thus avoiding abrasion. In digital recording apparatus, the head is in contact with the tape. The wear resistance problem in digital recording apparatus is overcome by the use of ferrite composite heads of calcium and barium titanate. Another solution is a thin film head in a substrate of, for example, titanium and aluminum oxide or tungsten carbide. These alternatives are expensive. Still another solution to the wear resistance problem is to provide the metal heads with a ceramic coating. A narrow window is provided in the ceramic to expose the core. Such structures are plagued with undercutting which causes the core to become irregular and exhibit signal degradation.
In general the wear problem in tape heads is largely solved by the use of ferrite composite heads. But, ferrite composite heads with separate read and write core structures (for read-while-write applications) have another problem: high cross feed between the write and read core structures, which leads to unacceptable error levels. For this reason, ferrite composite heads have been limited to a single gap operation whereby the entire tape is first written and then is read during a second pass of the tape to verify the accuracy of the written data. This double pass is time consuming, inefficient in tape storage density, and requires a considerable storage overhead of error correction codes. Whereas two separate passes are acceptable for low capacity storage devices (e.g., a floppy disk with 340 KB storage), two passes are unacceptable for high capacity drives (e.g., those that store 20 MB to 500 MB such as the 3M cartridges and the IBM 3480 cartridges).
The magnetic head structure of this invention makes possible the construction of read-while-write ferrite composite heads having separate read and write core structures on the same tape track. Cross feed is acceptably low; azimuth errors (which plague conventional ferrite head structures) are overcome; and the structure of this invention lends itself to automatic production and thus low manufacturing cost. In this connection, azimuth errors occur when adjacent gaps lie in planes which are not parallel to one another.